src: refactor HasWriteQueue() away

• process: use more direct sync I/O for stdio

  This avoids routing writes through the full LibuvStreamWrap write machinery. In particular, it enables the next commit, because otherwise the callback passed to _write() would not be called synchronously for pipes on Windows (because the latter does not support uv_try_write(), even for blocking I/O).

• src: remove HasWriteQueue()

  Tests are passing without it now, and this otherwise makes the code harder to reason about because the async flag on the write request object would not be set even though the callback would still be pending.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

src/stream_base

/cc @apapirovski